This invention relates to recycling methods, and more particularly to recycling of thermoset materials such as rubber.
A need exists for efficient and cost-effective methods for recycling thermoset materials. Because they undergo irreversible chemical changes, termed cross-linking, during processing, thermoset materials are difficult to recycle by conventional methods. Thermosets are generally used in high performance applications, such as in composite materials and tires, and include polyesters, polyurethanes, phenolics, melamines, epoxies, and rubbers.
By volume, the majority of thermosetting polymers comprises the chemically cross-linked rubbers used in tires. Rubber tires also make up a large volume of polymer waste today; in the United States alone, for example, there are an estimated two to three billion tires currently piled up in landfills, with another 250 million tires being discarded annually. In addition to taking up space in landfills, piles of discarded tires represent a major fire and health hazard.
Rubber is among the most difficult of all polymers to recycle, as rubbers have very high molecular weights and are chemically cross-linked, both of which factors render them insoluble and non-meltable. They are also filled with submicron-sized carbon black, which is nearly impossible to separate from the rubber.
Currently, there are few, primarily low technology, uses for materials reclaimed from scrap rubber tires. For example, rubber tires have been cut into sections or strips for use as flooring, mats, vibration and abrasion pads, and shoe soles. Scrap rubber and whole tires have also been burned as a fuel, but their use in this manner is also a source of pollution, since tires generally also include potentially toxic additives in addition to the rubber.
Another method of disposing of tires involves grinding the tires. In a typical grinding process, the steel and polymer fiber reinforcements are recovered from the tires, and the rubber is converted into either a fine particulate, called powder, or a very coarse particulate called crumb. These particulate rubber particles have been used as fillers in asphalt, cements, and roofing materials. Efforts have been made to blend tire powder or crumb into virgin rubber (e.g., at loadings of about 5-10%), but such processes require careful quality control as several different types of rubber are often present in a single tire, and they are not necessarily compatible with one another. For example, the tread, sidewall and inner lining materials usually have different formulations. Crumb rubber has also been used as a bedding material for cattle and hogs, as a replacement for sand in children""s playgrounds, and as a component in flooring for athletic tracks and related applications. There are several companies in America grinding tires using either an ambient temperature solvent based process or a cryogenic dry process.
The invention provides new methods for recycling thermoset materials such as natural rubbers, synthetic rubbers, silicone rubbers, and other elastomers and cross-linked polymers (e.g., isoprene rubbers; butyl rubbers; ethylene-propylene-diene rubbers, xe2x80x9cEPDMxe2x80x9d; nitrile, or acrylonitrile butadiene rubbers, xe2x80x9cNBRxe2x80x9d; styrene-butadiene rubbers, xe2x80x9cSBRxe2x80x9d; hard rubbers such as EBONITEX(copyright); mixtures of vulcanized rubbers from discarded tires). The invention is based on the discovery that by combining powdered or particulate thermoset materials with lubricants such as aromatic or paraffinic rubber processing oils or volatile solvents, the thermoset materials can be recycled under moderate temperature and pressure conditions to rapidly produce materials having physical properties nearing those of virgin thermoset materials. The resulting materials can, for example, be extruded or compaction molded into new shapes such as panels. The new recycling methods can also be carried out in either batch or continuous processes.
According to the new methods, rubber particles subjected to conditions of approximately 15 MPa (about 2200 psi) and 200xc2x0 C. for one hour form a single piece of rubber with excellent mechanical properties similar to those of virgin rubber. In addition, a solid part can be obtained in 5 minutes under a temperature of 180xc2x0 C. at about 1500 psi.
In general, the invention features methods for recycling thermoset materials such as rubber. The methods include the steps of adding a lubricant (e.g., a material such as an aromatic oil, a paraffinic oil, a volatile solvent, or a combination of such materials, that makes the thermoset material more processible; the added lubricant can remain in the rubber after processing or can be extracted during or after processing) to thermoset material to be recycled to obtain a lubricated material; and heating the lubricated material under an elevated pressure to obtain the recycled thermoset material.
The thermoset material to be recycled can include, for example, one or more of EPDM rubber, SBR rubber, NBR rubber, natural rubber, silicone rubber, isoprene rubber, and butyl rubber. The rubber can be vulcanized (e.g., cross-linked, particularly via sulfurxe2x80x94sulfur bonds), such as vulcanized rubber derived from discarded tires.
The rubber to be recycled can be, for example, in the form of a powder (e.g., particulate or crumb rubber).
The heating step can be conducted, for example, while the lubricated material is under a pressure of up to 10,000 psi or higher (e.g., 250 psi, 500 psi, 1,000 psi, 1,500 psi, 2,000 psi, 2,500 psi, 3,000 psi, 5,000 psi, 7,500 psi, 10,000 psi, or intermediate pressures such as 500 to 2,500 psi). The temperature can be, for example, between about 100xc2x0 C. and the decomposition temperature of the rubber (e.g., 100xc2x0 C., 150xc2x0 C., 180xc2x0 C., 200xc2x0 C., 220xc2x0 C., 230xc2x0 C., 240xc2x0 C., 250xc2x0 C., or higher, or intermediate pressures such as 180-220xc2x0 C.). The duration of heating under pressure can be, for example, from about 20 seconds to about 8 hours, e.g., 1 minute, 5 minutes, 10 minutes, 20 minutes, 30 minutes, 60 minutes, 2 hours, 3 hours, or longer, or intermediate times such as 1 to 60 minutes).
Another aspect of the invention features a process for making solid objects (e.g., hard or soft objects, including foam objects) from previously vulcanized rubber. The method includes the steps of obtaining vulcanized rubber ground into a powder; mixing the rubber powder with a lubricant (e.g., aromatic oils, paraffinic oils, and/or volatile solvents) to produce a rubber paste; and feeding the rubber paste into an extruder. The extruder heats the paste under pressure and then extrudes the paste to produce the solid or foam objects.
The process can, for example, be a continuous process. In such a continuous process, the solid objects initially formed can be, for example, green parts, and the process can also include heating the green parts in a post-annealing step (e.g., at ambient or elevated pressure) for a sufficient time (e.g., 20 minutes, 30 minutes, 1 hour, 2 hours, or longer) to produce hardened objects. The paste can, for example, be extruded into a mold, which can optionally be heated. The process can be a ram extrusion process.
The recycled rubber and objects prepared according to these methods and processes are also considered to be an aspect of the invention.
The invention provides several advantages. For example, generally require moderate pressures and temperatures. The new methods can make use of commercially available rubber powder. Since powder obtained by grinding tires already contains the ultra-fine carbon black fillers required to reinforce rubber materials, the use of the powder in the new methods eliminates a processing step.
In the new methods, one minute or less can be adequate time for obtaining a xe2x80x9cgreenxe2x80x9d part that can then be post-treated after molding at elevated temperatures for longer periods of time at ambient pressure, even without any powder preheating. The rapidity of production of green parts in the new methods makes them amenable to use in continuous recycling processes, whereas many earlier methods were batch processes.
The new methods allow production of homogeneous materials with excellent mechanical properties.
Since the paste formed by combining rubber powder and extender oil is more liquid-like than rubber powders used alone, even complicated geometries can be molded with relative ease. The new methods also allow used rubbers to be recycled into usable parts such as rubber sheeting, wheelbarrow, pushcart, or bicycle tires, and molded goods, without size limitations. The use of lubricants in the recycling process can also eliminate the need to add plasticizers to the compositions. When thermosets such as vulcanized rubber are ground into a powder, plasticizers can be lost. Also, plasticizers can be lost during the lifetime of rubber products. The addition of lubricants in the new methods can effectively replenish the plasticizers, and thus result in materials having properties similar to virgin thermosets. In the case of rubber powders from old tires, for example, the oil content is generally quite low due to the fact that the oils slowly escape over time, and the use of extender oils as lubricants in the new methods can result in recycled rubber having a composition similar to that of virgin rubber.
Using the methods of the invention, tires can be patched and retreads can be attached to tire bodies by applying heat and pressure, for example, without requiring adhesives.
By allowing used thermoset materials such as discarded tires to be recycled, the new methods can reduce the amount of waste discarded into landfills, lower production costs, reduce energy consumption, and decrease pollution.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.